Copy performance fees (Phase 4)

Money-manager performance fees on a high-water-mark, the standard copy-trading model (cTrader Copy, Darwinex, ZuluTrade profit-share): a provider charges a percentage of new profit above each follower's peak equity — never on the opening balance, and never twice for ground already recovered. Opt-in via App:Copy:FeesEnabled (off by default).

The model (high-water-mark)

Per destination (follower account), each settlement:

1. First settlement seeds the high-water-mark (HWM) at the current equity → no charge (a follower is never billed on their deposit).
2. New high (equity > HWM): fee = performanceFeePercent × (equity − HWM), then HWM ← equity.
3. At or below the peak: no fee, HWM unchanged — the follower must first recover past the old peak, so they are never charged twice for the same gains.

The fee arithmetic is a domain invariant on CopyDestination.SettleFee(equity) — the aggregate owns it; the settlement service only supplies the polled equity and records the returned amount. PerformanceFee is a value object capped at 50% so a misconfiguration can't charge away a follower's whole gain.

How it settles

CopyFeeSettlementService (BackgroundService, only when FeesEnabled)
   │  every App:Copy:FeeSettlementInterval
   ├─ load running profiles with a fee-configured destination
   ├─ ICopyEquityReader.ReadEquityAsync(ctid)   ← OpenApiCopyEquityReader opens a session,
   │                                               computes balance + floating P&L (PropFirmEquityCalculator)
   ├─ destination.SettleFee(equity)             ← HWM logic on the aggregate
   └─ persist advanced HWM + append CopyFeeAccrual (only on a new high)

• ICopyEquityReader is a Core abstraction; the live implementation (OpenApiCopyEquityReader) is the only infra piece — so the settlement + HWM logic is exercised in tests with a fake reader, no live broker.
• CopyFeeAccrual is an append-only log (HWM-before, equity, fee %, fee amount, settled-at) — a fact log for the fee report and billing, not an aggregate.

Configuration & API

App:Copy setting	Default	Effect
FeesEnabled	false	Run the settlement service.
FeeSettlementInterval	1h	How often equity is polled and fees settled.

Per-destination: PerformanceFeePercent (0–50) is set on the destination (add/edit destination request).

• GET /api/copy/profiles/{id}/fees — the profile's fee accruals + total charged.

Tests

• Unit (CopyPerformanceFeeTests) — the HWM invariant: first settlement seeds + charges nothing; a new high charges only the gain above the peak; at/below the peak charges nothing and the peak never retreats; after a drawdown only the recovery past the old peak is charged; 0% never charges; the VO rejects out-of-range percents.
• Integration (CopyFeeSettlementTests, real Postgres, fake equity reader) — seed→10k (no charge, mark seeded), 12k (charges 400, mark advances), 11k (no charge, mark held); accrual persisted with the right owner/amount.

The copy host is untouched by fees (settlement is a separate DB job), so the copy DST stress suite is unaffected (23/23).